If you arrive in Thessaloniki most people take a bus to Ouranopolis (does anyone ever took a taxi?). It is only a short taxi trip from the airport to the K-tel busstation: on this site you can check the time table , but I’ll also show the time table here.
I found a good site from American students living Thessaloniki who published recent and usefull information.
A good idea might be the following: if have the opportunity to get your Diamonitirion the day before you plan the visit Athos at the Pilgrim’s Bureau in Ouranopolis (before 2:00!), you are able to go with the first ferry boat, which departs at 6:30 AM, in stead of taking the 9:45 AM boat: this will save you about 3 hours and you have much more possbilities to wander around and to get to the place where you planned to sleep (that is to say, if like walking, if you plan to use motorized transportation, just stay in bed a little longer!)
Wim Voogd, 2-9